1. Field of the Invention
The present invention relates to improvements in the choice of transition metal, ligands, counterions and initiators in the atom or group transfer radical polymerization process, and new polymeric products produced thereby.
2. Discussion of the Background
Living polymerization systems have been developed which allow for the control of molecular weight, end group functionality, and architecture. [Webster, O. Science, 1991, 251 887] Most notably, these systems involve ionic polymerization. As these polymerization systems are ionic in nature, the reaction conditions required to successfully carry out the polymerization include the complete exclusion of water from the reaction medium. Another problem with ionic living polymerizations is that one is restricted in the number of monomers which can be successfully polymerized. Also, due to the high chemoselectivity of the propagating ionic centers, it is very difficult, if not impossible, to obtain random copolymers of two or more monomers; block copolymers are generally formed.
Radical polymerization is one of the most widely used methods for preparing high polymer from a wide range of vinyl monomers. Although radical polymerization of vinyl monomers is very effective, it does not allow for the direct control of molecular weight (DPn≠Δ[Monomer]/[Initiator]o), control of chain end functionalities or for the control of the chain architecture, e.g., linear vs. branched or graft polymers. In the past five years, much interest has been focused on developing a polymerization system which is radical in nature but at the same time allows for the high degree of control found in the ionic living systems.
A polymerization system has been previously disclosed that does provide for the control of molecular weight, end groups, and chain architecture, and that was radical in nature. (Matyjaszewski, K.; Wang, J.-S. Macromolecules 1995, 28, 7901; Matyjaszewski, K.; Patten, T.; Xia, J.; Abernathy, T. Science 1996, 272 866; U.S. patent application Ser. No. 08/414,415, now U.S. Pat. No. 5,763,548; Ser. No. 08/559,309, now U.S. Pat. No. 5,807,937; 08/677,828, now U.S. Pat. No. 5,789,487) the contents of which are hereby incorporated by reference. This process has been termed atom transfer radical polymerization, ATRP. ATRP employs the reversible activation and deactivation of a compound containing a radically transferable atom or group to form a propagating radical (R•) by a redox reaction between the radical and a transition metal complex (Mtn+1) with a radically transferable group (X), Scheme 1.

Controlled polymerization is initiated by use, or formation, of a molecule containing a radically transferable atom or group. Previous work had concentrated on the use of an alkyl halide adjacent to a group which can stabilize the formed radical. Other initiators may contain inorganic/pseudo halogen groups which can also participate in atom transfer, such as nitrogen, oxygen, phosphorous, sulfur, tin, etc.
The most important aspect of the reaction outlined in Scheme 1 is the establishment of an equilibrium between the active radicals and the dormant species, R—X (dormant polymer chains=Pn—X). Understanding and controlling the balance of this equilibrium is very important in controlling the radical polymerization. If the equilibrium is shifted too far towards the dormant species, then there would be no polymerization. However, if the equilibrium is shifted too far towards the active radical, too many radicals are formed resulting in undesirable bimolecular termination between radicals. This would result in a polymerization that is not controlled. An example of this type of irreversible redox initiation is the use of peroxides in the presence of iron (II). By obtaining an equilibrium which maintains a low, but nearly constant concentration of radicals, bimolecular termination between growing radicals can be suppressed, one obtains high polymer.
Using copper as an example of the transition metal that participates in the redox cycle, the rate of polymerization has been shown to obey the following rate law:
                              R          o                =                              (                          k              p                        )                    ⁢                                                    (                                  k                  eq                                )                            ⁡                              [                M                ]                                      ⁡                          [                              R                -                X                            ]                                ⁢                                    [                              M                t                n                            ]                                      [                              X                -                                  M                  t                                      n                    +                    1                                                              ]                                                          (        1        )            
When kp=rate constant of propagation; keq=equilibrium constant; [M], [R—X], [Mtn][Mtn+1-X] are the respective concentrations of monomer, initiator, transition metal compound and oxidized transition metal compound.
ATRP involves the use of low valent metal salts, Mn, e.g., copper (I), iron (II), etc., which can homolytically abstract an atom or group, X, from a functionalized compound to initiate the polymerization, as shown above in Scheme 1. Deactivation of the growing polymer chain by reaction with the now higher oxidation state metal, Mtn+1−X, reverts the metal back to its lower oxidation state and an oligomer with the X group at the chain end. This process repeats itself, re-initiating the polymer chain to begin propagation again. After numerous repetitions, high polymer can be obtained with DPn=Δ[M]/[I]o and Mw/Mn<1.5.
Previously, pure metals have been employed to prepare ill-defined polymers. Otsu has reported the use of nickel (0) in a “living” polymerization, but the resulting polymer had very broad polydispersities, Mw/Mn>2. [Otsu, T.; Tazaki, T.; Yohioka, M. Chemistry Express 1990, 5(10), 801] In fact, the resulting polymer had a bimodal distribution upon examination of the size exclusion chromatograms, indicating that this was not a controlled or “living” polymerization. This failure to obtain a controlled polymerization is most likely due to the slow deactivation of the propagating radical. It can also be attributed to nickel favoring the +2 oxidation state versus the +1 oxidation state. As such, nickel (0) would initiate the polymerization by formation of a radical and nickel (I). The nickel (I) would then preferentially react with an alkyl halide to form nickel (II) and a second radical. The back reaction to nickel (0) from nickel (I), or from nickel (II) to nickel (I), would not be favored, resulting in an uncontrolled radical polymerization. Additionally, as a side reaction, the radical may be reduced to an anion by nickel (I), forming nickel (II).
Others have reported the use of zero valent metals to initiate polymerization of vinyl metals, also without controlling the polymerization. [Bamford, C. H.; Duncan, F. J.; Reynolds, R. J. W.; Seddon, J. D. J. Polym. Sci. Part C 1968, 23, 419; Otsu, T.; Tazaki, T.; Yoshioka, M. Chemistry Express 1990, 5(10), 801; Otsu, T.; Aoki, S.; Nishimura, M.; Yamaguchi, M.; Kusuki, Y. J. Polym. Sci., Polym. Letters 1967, 5, 835; Otsu, T.; Yamaguchi, M.; Takemura, Y.; Kusuki, Y.; Aoki, A. J. Polym. Sci. Polym. Letters 1967, 5, 697]
Recently, it was reported that iron metal was successfully used catalytically in the addition of carbon tetrachloride or chloroform to alkenes. [Bellesia, F.; Forti, L.; Ghelfi, F.; Pagnoni, U. Synthetic Communications 1997, 27(6), 961; Forti, L.; Ghelfi, F.; Lancellotti, M. L.; Pagnoni, U. M. Synthetic Communications 1996, 26(9), 1699] Similar systems have been employed to prepare low molar mass polyhalogenated organic molecules. [Decker, D. L.; Moore, C.; Tousignant, W. G., U.S. Pat. No. 3,862,978, 1975; Kondakov, S. E.; Smirnov, V. V. Kinet. Catal. 1995, 36(3), 315] The mechanism was the same as that in atom transfer radical addition: only monoadducts or low molar mass organic molecules were prepared by insertion of the alkene in one of the carbon halogen bonds of carbon tetrachloride (or chloroform).